Filled skutterudite-based alloy, production method thereof and thermoelectric conversion device fabricated using the alloy

ABSTRACT

A method for producing a filled skutterudite-based alloy includes the steps of melting alloy raw material that includes a rare earth metal R that is at least one species selected from among La, Ce, Pr, Nd, Sm, Eu and Yb, a transition metal T that is at least one species selected from among Fe, Co, Ni, Os, Ru, Pd, Pt and Ag, and metallic antimony Sb to form a melt; and rapidly quenching the melt through strip casting to form a solidified product that is the filled skutterudite-based alloy advantageously usable for a thermoelectric element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dateof Provisional Application Ser. No. 60/410,792 filed Sep. 16, 2002pursuant to 35 U.S.C. §111(b), the contents of which are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a filled skutterudite-based alloy foruse in a thermoelectric conversion element, which converts heat directlyinto electricity on the basis of the Seebeck effect, to a method forproducing the alloy and to a thermoelectric conversion elementfabricated using the alloy.

BACKGROUND ART

Thermoelectric conversion materials formed of a filledskutterudite-based alloy have low thermal conductivity, as compared withan intermetallic compound, such as CoSb₃, having a skutterudite-typecrystal structure, which compound is a type of conventionalthermoelectric conversion materials. Therefore, such thermoelectricmaterials formed of a filled skutterudite-based alloy show promise asthermoelectric conversion materials for use particularly in ahigh-temperature range.

A filled skutterudite-based alloy is an intermetallic compoundrepresented by the formula RT₄Pn₁₂ (wherein R represents a rare earthmetal, T a transition metal, and Pn an element, such as P, As or Sb). Inthe alloy, interstitial spaces present in skutterudite-type crystalsrepresented by formula TPn₃ (wherein T represents a transition metal,and Pn an element, such as P, As or Sb), are partially filled withlarge-mass atoms, such as rare earth metals (R). One reason whythermoelectric conversion materials formed of a filledskutterudite-based alloy have low thermal conductivity is that, wheninterstitial spaces included in the skutterudite-type crystals arefilled with rare earth metal elements, the rare earth metal elementscause vibration, by virtue of weak bonding between the elements and Pn,thereby providing phonon scattering centers.

Appropriate selection of R or T is considered to allow selectiveconversion of the filled skutterudite-based alloy into either a p-typematerial or an n-type material. Thus, in order to select the p-type orn-type, attempts have been made to substitute elements, such as Co andNi, for part of component T comprising Fe atoms.

The thus produced p-type and n-type filled skutterudite-based alloys areshaped into blocks, and a p-type block and an n-type block are directlyor indirectly (i.e., by the mediation of a metallic conductor) joinedtogether so as to form a p-n junction, whereby a thermoelectricconversion element can be fabricated. Alternatively, a thermoelectricconversion element module (U- or V-shape) can be fabricated byconnecting p-type and n-type filled skutterudite-based alloythermoelectric conversion members so as to form a p-n junction. Asanother alternative, a series of thermoelectric conversion elementshaving a p-n junction are connected and equipped with a heat exchangerto thereby provide a thermoelectric conversion system, through whichelectricity can be generated on the basis of a difference intemperature.

Conventionally, thermoelectric conversion elements have been fabricatedby use of a filled skutterudite-based alloy in such a manner ascomprising the steps of weighing high-purity powder materials of a rareearth metal, a transition metal, P, As, Sb, etc. so as to attain thecomposition of a target filled skutterudite alloy, mixing the materials,calcining the mixture at 800° C. or lower, pulverizing the calcinedproduct, hot-press-sintering or plasma-discharge-sintering thepulverized product by heating to 800° C. and cutting the sinteredproduct.

However, when the above method is employed, the crystal grain size ofthe formed filled skutterudite-based alloy is greatly affected by theconditions of material powder. In addition, there arises a problem thatan increase in crystal grain size, which tends to occur when sinteringconditions are not strictly controlled, causes a deterioratedperformance of the fabricated thermoelectric conversion elements.

In order to avoid the above problem, there has been proposed a techniquewhere a sintered product of Sb-containing skutterudite-basedthermoelectric material, which is a type of filled skutterudite-basedthermoelectric conversion material, is formed from minuteskutterudite-structure crystal grains and a metal oxide is dispersed inthe grain boundaries of the crystal grains (JP-A 2000-252526).

The publication discloses that the use of the above technique reducesthe mean crystal grain size of the skutterudite-structure crystal grainsto 20 μm or less. However, the method has a problem that the presence ofmetal oxide in the crystal grain boundaries lowers electricconductivity.

Another method for producing a thermoelectric conversion material formedof a filled skutterudite-based alloy is a heat treatment of ribbonsfabricated through the melt-spinning method (JP-A 2002-26400). Themelt-spinning method generally includes pouring a molten metal underpressure onto a roller that is rotating at high speed, from a nozzleformed of a quartz-made tube having a hole of approximately 1 mm in itstip.

However, even when the method is employed, a filled skutteruditethermoelectric conversion element having a satisfactory purity isdifficult to obtain since the produced alloy ribbons assume amorphous orcontain decomposition products, such as Sb₂Fe and Sb. Thus, the alloyribbons must be heated at 873 K to 1,073 K for five hours or longer soas to attain a practically usable purity, thereby constituting anotherproblem.

Furthermore, in any of the aforementioned methods, when steps from amaterial preparation step to a sintering step are carried out in anoxygen-containing atmosphere, such as air, rare earth metal atoms areremoved from the crystal lattice of a filled skutterudite structure bythe oxidation of rare earth metals, resulting in partial decompositionof the skutterudite structure to form Sb₂Fe and Sb, which is alsoproblematic.

One object of the present invention is to provide a method for producinga filled skutterudite-based thermoelectric conversion material withoutrequiring adoption of an alloy-pulverizing step and a pulverizedproduct-sintering step.

Another object of the invention is to provide a filledskutterudite-based alloy advantageously usable for a thermoelectricconversion element without being modified.

Still another object of the invention is to provide a thermoelectricconversion element fabricated using the above filled skutterudite-basedalloy.

DISCLOSURE OF THE INVENTION

The present invention provides a method for producing a filledskutterudite-based alloy comprising melting alloy raw materialcomprising a rare earth metal R that is at least one species selectedfrom among La, Ce, Pr, Nd, Sm, Eu and Yb, a transition metal T that isat least one species selected from among Fe, Co, Ni, Os, Ru, Pd, Pt andAg, and metallic antimony Sb to form a melt; and rapidly quenching themelt through strip casting to form a solidified product.

In the method, the alloy raw material is melted at a temperature of 800to 1,800° C., and the melt is rapidly quenched at a cooling rate of 10²to 10⁴° C./second, as measured within a range of a temperature of themelt to 800° C.

In the method, the alloy raw material is melted in an inert gasatmosphere at a pressure higher than atmospheric pressure of 0.1 MPa andnot higher than 0.2 Mpa.

In the method, the solidified product comprises alloy strips having athickness of 0.1 to 2.0 mm.

The present invention also provides a filled skutterudite-based alloyproduced through the method mentioned above, that contains a filledskutterudite phase in an amount of at least 95 mass %;

In the filled skutterudite-based alloy, the filled skutterudite-basedalloy contains a filled skutterudite phase in an amount of at least 95vol. %, and the alloy further contains a phase, other than the filledskutterudite phase, having a maximum diameter of 10 μm or less.

The filled skutterudite-based alloy contains oxygen, nitrogen and carbonin a total amount of 0.2 mass % or less.

The invention also provides a thermoelectric conversion elementfabricated using the filled skutterudite-based alloy mentioned above.

The present invention adopting the strip casting as described aboveenables easy mass production of alloys comprising a substantiallyhomogenous filled skutterudite phase, resulting in a great decrease inproduction cost.

The filled skutterudite-based alloy can be produced without beingsubjected to pulverizing and sintering steps and therefore hassatisfactory mechanical strength and can be easily worked for producinga thermoelectric conversion element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a strip casting production apparatusemployed in the present invention.

FIG. 2 is an X-ray diffraction chart of a LaFe₄Sb₁₂ filledskutterudite-based alloy produced in the present invention.

FIG. 3 is a back-scattered electron image of the cross-section of theLaFe₄Sb₁₂ filled-skutterudite-based alloy produced in the presentinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

The filled-skutterudite-based alloy according to the present inventioncontains, in an amount of at least 95 vol. %, a filled skutteruditephase represented by the formula RT₄Sb₁₂, wherein R stands for at leastone species selected from among La, Ce, Pr, Nd, Sm, Eu and Yb, and T forat least one species selected from among Fe, Co, Ni, Os, Ru, Pd, Pt andAg. Sb may be partially substituted by As or P.

Examples of the rare earth metal R which can be used as a raw materialfor producing the filled skutterudite-based alloy of the presentinvention include a rare earth metal (purity: 90 mass % or higher, thebalance being unavoidable impurities, such as Al, Fe, Mo, W, C, O andN), and a misch metal comprising Ce and La (rare earth metal content: 90mass % or higher, the balance being unavoidable impurities, such as Al,Fe, Mo, W, C, O and N). Examples of the transition metal T that can beused include pure iron (purity: 99 mass % or higher) and othertransition metals, such as Co and Ni (purity: 99 mass % or higher).Examples of Sb that can be used include metallic antimony (purity: 95mass % or higher, the balance being unavoidable impurities, such as Pb,As, Fe, Cu, Bi, Ni, C, O and N). The raw material for producing thefilled skutterudite-based alloy of the present invention is prepared byweighing these components (R, T and metallic antimony) so as to adjustthe alloy composition to RT₄Sb₁₂. The compositional proportions of rawmaterials (R, T and Sb) for producing the alloy of the present inventionpreferably fall within ranges of 7.5 to 8.3 mass %, 12.1 to 12.3 mass %,and 79.5 to 80.2 mass %, respectively.

According to the present invention, the filled skutterudite-based alloyis produced through the strip casting method (SC method). FIG. 1 showsan apparatus employed for producing the alloy through the SC method. InFIG. 1, reference numerals 1, 2, 3, 4, 5 and 6 denote a crucible, atundish, a copper roller, a receiving box, a molten alloy and asolidified alloy strip, respectively.

According to the method for producing the filled skutterudite-basedalloy of the present invention, the alloy raw material that has beenprepared in the aforementioned manner is melted in the crucible 1 at 800to 1,800° C. in an atmosphere of inert gas, such as Ar or He. In thiscase, the pressure of the atmosphere is preferably controlled to apressure higher than atmospheric pressure (0.1 MPa) and no higher than0.2 MPa, in view of the fact that the amount of vaporized Sb can besuppressed.

The molten alloy 5 prepared by melting the alloy raw material is pouredvia the tundish 2 onto the copper roller 3, which is cooled with waterand is rotating in a direction indicated by the arrow shown in FIG. 1,to thereby rapidly quench the alloy. During this process, the coolingrate, as measured within a temperature range of the temperature of themolten alloy to 800° C., is preferably controlled to 10² to 10⁴°C./second in order to attain a metallographic structure of the alloyformed of a uniform filled skutterudite phase, more preferably to 5×10²to 3×10³° C./second. The molten alloy-cooling rate can be controlled toa desired value by modifying the rotating speed (as represented byperipheral velocity) of the copper roller 3 or by modifying the amountof the molten alloy poured onto the copper roller 3.

The solidified alloy is removed from the copper roller 3 in the form ofstrips 6, which are collected into the receiving box 4. The thuscollected strips are cooled in the receiving box 4 to room temperatureand then removed from the box. In this case, the rate of cooling thesolidified alloy strips can be controlled through thermal insulation orforced cooling of the receiving box 4. By controlling the rate ofcooling the thus solidified alloy strips, uniformity in the filledskutterudite phase present in the alloy can be further enhanced.

The filled skutterudite-based alloy strips produced through the SCmethod according to the present invention preferably have a thickness of0.1 to 2.0 mm. Controlling the thickness of the alloy strips to 0.1 to2.0 mm yields a filled skutterudite-based alloy that has satisfactorymechanical strength and can be easily worked for producing athermoelectric conversion element.

The filled skutterudite-based alloy of the present invention produced inthe aforementioned manner exhibits a maximum peak intensity, attributedto the filled skutterudite phase, of 95% or higher as determined throughidentification of formed phases on the basis of the powder X-raydiffraction method, when the alloy has been removed from a productionapparatus employed in the SC method and has not undergone any furtherheat treatment. FIG. 2 shows an identification feature of phases formedin the filled skutterudite-based alloy of the present invention throughthe powder X-ray diffraction method.

FIG. 2 shows X-ray diffraction measurement results of the alloy that hasbeen removed from a production apparatus employed in the SC method andthen pulverized without affording any further treatment thereto. Thefilled skutterudite phase content can be determined by calculating theintegral intensity of the maximum peak attributed to the filledskutterudite phase, calculating the integral intensity of the maximumpeak attributed to each of the phases (e.g., Sb₂Fe and Sb) other thanthe filled skutterudite phase and dividing the integral intensity forthe filled skutterudite phase by the sum of the integral intensity forthe filled skutterudite phase and the integral intensities for the otherphases. Specifically, the filled skutterudite phase accounts for 99 mass% or more of the alloy, as determined from the X-ray diffraction chartshown in FIG. 2.

The filled skutterudite-based alloy of the present invention produced inthe aforementioned manner contains a filled skutterudite phase in anamount of at least 95 vol. %, and a phase other than the filledskutterudite phase in an amount of 5 mass % or less. It should be notedthat the phase other than the filled skutterudite phase includes aphase, such as of Sb₂Fe or Sb. In the alloy of the present invention,each of the phases other than the filled skutterudite phase has amaximum diameter of 10 μm or less.

The ratio by volume of the amount of the filled skutterudite phasecontained in the alloy to that of a phase other than the filledskutterudite phase can be determined by calculating the ratio of the“area of the filled skutterudite phase” to the “area exhibiting contrastdiffering from that of the filled skutterudite phase” on the basis of aback-scattered electron image observed under a scanning electronmicroscope. In addition, the maximum diameter of the phase other thanthe filled skutterudite phase can be determined from the back-scatteredelectron image. FIG. 3 shows an example of the back-scattered electronimage of the filled skutterudite-based alloy of the present inventionobserved under a scanning electron microscope. As is clear from theimage, the alloy contains a virtually uniform filled skutterudite phasein an amount of 95 vol. % or more, and the phase other than the filledskutterudite phase has a maximum diameter of 10 μm or less.

According to the present invention, melting and casting can be performedin an inert atmosphere. Thus, the total amount of oxygen, nitrogen andcarbon contained in the filled skutterudite-based alloy of the presentinvention can be suppressed to 0.2 mass % or less.

Upon production of a thermoelectric conversion element, the filledskutterudite-based alloy of the present invention is suitably used as ap-type material. Conventional substances, such as Pb-Te-based material,may be used in combination with the filled skutterudite-based alloy, asan n-type material. A p-type thermoelectric conversion member and ann-type thermoelectric conversion member are directly or indirectly(i.e., by the mediation of a metallic conductor) joined together tothereby fabricate a thermoelectric conversion element having a p-njunction. When a thermoelectric element module is fabricated, the alloyof the present invention can be used in combination with a Bi—Te-basedmaterial or Se-based compound that has excellent low-temperaturecharacteristics or with a Co oxide-based compound that has excellenthigh-temperature characteristics.

The present invention will next be described in more detail withreference to Examples.

EXAMPLE 1

Metallic La that is rare earth metal, electrolytic iron and Sb wereweighed such that a stoichiometric composition of LaFe₄Sb₁₂ wasattained. The mixture was melted in an Ar atmosphere at 0.1 MPa byheating it to 1,400° C. Subsequently, by means of the strip castingapparatus shown in FIG. 1, the molten alloy was poured onto the copperroller, which was cooled with water and was rotating at a rotating speedof 0.92 m/s, at a pour rate of 150 g/s and a pour width of 85 mm tothereby produce alloy strips having a thickness of 0.28 mm. The coolingrate at the time of casting is estimated to be approximately 1×10³°C./sec.

The thus produced alloy strips were pulverized, and the formed powderwas analyzed through powder X-ray diffractometry. As shown in FIG. 2,almost no peak attributed to Sb₂Fe or Sb was observed. The filledskutterudite phase content, as calculated on the basis of the chart, wasfound to be 98% or more (as LaFe₄Sb₁₂), and the Sb₂Fe content was foundto be 2% or less.

The thus produced alloy strips were further subjected to heat treatmentat 550° C. for one hour in an Ar flow at atmospheric pressure. PowderX-ray diffractometry revealed that the heat-treated product has a filledskutterudite (LaFe₄Sb₁₂) phase content of approximately 100%. Themetallographic microstructure and formed phases of the heat-treatedalloy were confirmed by back-scattered electron images, and the resultsindicated that no phase separation was identified and that almost theentirety of the alloy was formed of a uniform filled-skutterudite phase.

EXAMPLE 2

Misch metal that is rare earth metal consisting of 53 mass % of Ce and47 mass % of La, electrolytic iron and Sb (99%) were weighed such that astoichiometric composition of (Ce_(x), La_(1-x))Fe₄Sb₁₂ was attained.The mixture was melted in an Ar atmosphere at 0.1 MPa by heating it to1,400° C. Subsequently, by means of the strip casting apparatus shown inFIG. 1, the molten alloy was poured onto the copper roller, which wascooled with water and was rotating at a rotating speed of 0.92 m/s, at apour rate of 150 g/s and a pour width of 85 mm to thereby produce alloystrips having a thickness of 0.28 mm.

The thus produced alloy was pulverized, and the formed powder wasanalyzed through powder X-ray diffractometry. The results indicated thatthe filled skutterudite phase content, calculated from maximum peakintensities, is 98% or more (as (Ce_(x), La_(1-x))Fe₄Sb₁₂), and theSb₂Fe content is 2% or less.

Immediately after the completion of casting the alloy, the cooling ratein the receiving box was adjusted, in an Ar atmosphere at atmosphericpressure, to 2° C./sec in a temperature range of 700° C. to 500° C.Powder X-ray diffractometry revealed that the product has a filledskutterudite ((Ce_(x), La_(1-x))Fe₄Sb₁₂) phase content of 99% or more.The metallographic microstructure and formed phases of the heat-treatedalloy were confirmed by back-scattered electron images, and the resultsindicated that no phase separation was identified and that almost theentirety of the alloy was formed of a uniform filled skutterudite phase.

EXAMPLE 3

Metallic La that is rare earth metal, electrolytic iron and Sb wereweighed such that a stoichiometric composition of LaFe₄Sb₁₂ wasattained. The mixture was melted in an Ar atmosphere at 0.2 MPa byheating it to 1,400° C. Subsequently, by means of the strip castingapparatus shown in FIG. 1, the molten alloy was poured onto the copperroller, which was cooled with water and was rotating at a rotating speedof 0.92 m/s, at a pour rate of 150 g/s and a pour width of 85 mm tothereby produce alloy strips having a thickness of 0.28 mm.

The thus produced alloy strips were pulverized, and the formed powderwas analyzed through powder X-ray diffractometry. The results indicatedthat the filled skutterudite phase content, calculated from maximum peakintensities, is 95% or more (as LaFe₄Sb₁₂), and the Sb₂Fe content is 5%or less.

The thus produced alloy strips were further subjected to heat treatmentat 550° C. for one hour in an Ar flow at atmospheric pressure. PowderX-ray diffractometry revealed that the heat-treated product has a filledskutterudite (LaFe₄Sb₁₂) phase content of 99% or more. Themetallographic microstructure and formed phases of the heat-treatedalloy were confirmed by back-scattered electron images, and the resultsindicated that no phase separation was identified and that almost theentirety of the alloy was formed of a uniform filled skutterudite phase.

COMPARATIVE EXAMPLE 1

Metallic La that is rare earth metal, electrolytic iron and Sb wereweighed such that a stoichiometric composition of LaFe₄Sb₁₂ wasattained. The mixture was melted in a reduced pressure atmosphere of 10Pa by heating it to 1,400° C. While the reduced pressure conditions weremaintained, the molten alloy was poured onto a copper roller, which wascooled with water and was rotating at a rotating speed of 0.92 m/s, at apour rate of 150 g/s and a pour width of 85 mm to thereby produce castalloy strips having a thickness of 0.28 mm, in the same manner as inExample 1.

The thus produced alloy was pulverized, and the formed powder wasanalyzed through powder X-ray diffractometry. The results indicated thatdiffraction peaks were attributed almost entirely to Sb₂Fe and Sb. Themetallographic microstructure and formed phases of the heat-treatedalloy were confirmed by back-scattered electron images, and the resultsindicated that the alloy was formed of a plurality of phases. The alloywas found to have an oxygen concentration higher than 0.2 mass % and anSb content less than the stoichiometric level. Accordingly, formation ofthe filled skutterudite phase is considered to be impossible because ofremoval of the rare earth metal from the skutterudite phase andevaporation of the Sb during melting, resulting in deviation of thecomposition from the stoichiometry.

COMPARATIVE EXAMPLE 2

Metallic La that is rare earth metal, electrolytic iron and Sb wereweighed such that a stoichiometric composition of LaFe₄Sb₁₂ wasattained. The mixture was melted in an Ar atmosphere at 0.1 MPa byheating it to 1,400° C. Subsequently, the molten alloy was poured onto abook mold made of a copper plate (width: 10 mm, thickness: 20 mm) at apour rate of 150 g/s to thereby produce an alloy.

The thus produced alloy was pulverized, and the formed powder wasanalyzed through powder X-ray diffractometry. The results indicated thatdiffraction peaks were almost entirely attributed to Sb₂Fe and Sb. Thealloy was further subjected to heat treatment at 550° C. for one hour inan Ar flow at atmospheric pressure. Powder X-ray diffractometry revealedthat almost the entirety of the heat-treated product was still formed ofSb₂Fe and that the alloy had virtually no filled skutterudite phase. Themetallographic microstructure and formed phases of the heat-treatedalloy were confirmed by back-scattered electron images, and the resultsindicated that the alloy was formed of a plurality of phases. Althoughthe alloy was found to have an oxygen concentration of 0.1 mass % orless and an Sb amount almost equal to the stoichiometric level, forminga uniform filled skutterudite phase in the alloy might require heattreatment for a very long period of time.

INDUSTRIAL APPLICABILITY

According to the present invention, a filled skutterudite-based alloy ofalmost uniform metallographic structure can be produced in a largeamount and in a simple manner through the strip casting method. Thefilled skutterudite-based alloy produced through the method of thepresent invention can be used, without pulverization and sintering, forproducing a thermoelectric conversion element. Thus, cost of producingthermoelectric conversion elements can be greatly reduced.

1. A method for producing a filled skutterudite-based alloy containing acomposition of RT₄Sb₁₂, comprising: melting alloy raw materialcomprising a rare earth metal R that is at least one species selectedfrom among La, Ce, Pr, Nd, Sm, Eu and Yb, a transition metal T that isat least one species selected from among Fe, Co, Ni, Os, Ru, Pd, Pt andAg, and metallic antimony Sb at a temperature of 800 to 1,800° C. toform a melt; and rapidly quenching the melt at a cooling rate of 5×10²to 3×10³° C./second, as measured within a range of the temperature ofthe melt to 800° C. through strip casting wherein the rapid quenchingforms a solidified alloy containing a filled skutterudite phase in anamount of at least 95 mass %; and collecting the solidified alloy into areceiving box.
 2. The method according to claim 1, wherein the alloy rawmaterial is melted in an inert gas atmosphere at a pressure higher thanatmospheric pressure of 0.1 MPa and not higher than 0.2 MPa.
 3. Themethod according to claim 1, wherein the solidified product comprisesalloy strips having a thickness of 0.1 to 2.0 mm.
 4. A filledskutterudite-based alloy produced through the method according to claim1, that contains a filled skutterudite phase in an amount of at least 95mass %.
 5. The filled skutterudite-based alloy according to claim 4,wherein it contains a filled skutterudite phase in an amount of at least95 vol. % and further contains a phase, other than the filledskutterudite phase, having a maximum grain diameter of 10 μm or less. 6.The filled skutterudite-based alloy according to claim 4, wherein itcontains oxygen, nitrogen and carbon in a total amount of 0.2 mass % orless.
 7. A thermoelectric conversion element fabricated using the filledskutterudite-based alloy according to claim
 4. 8. The method accordingto claim 1, wherein the receiving box is cooled at a rate of 2°C./second at a temperature within the range of from 700° C. to 500° C.